1. Field of the Invention
The present invention relates to a multi-tube type heat transfer apparatus in which a heat-exchange medium flows on the side of a shell of the apparatus to perform cooling or heating of heat transfer tubes, which apparatus is available, for instance, in a multi-tube type acrylic acid reactor or in a multi-tube type heat-exchanger.
2. Description of the Prior Art
In the following, description will be made of a multi-tube type heat transfer apparatus in the prior art, by way of example, in connection to an acrylic acid reactor.
Heretofore, in the synthesis of acrylic acid by oxidizing propylene, propylene is subjected to catalytic oxidation at a high temperature in gaseous phase under existence of steam in a tubular reactor packed with molybdenum group catalyst to produce acrolein. Subsequently acrolein is oxidized into acrylic acid, and in order to remove the heat of reaction generated at that time and also to effectively utilize the heat, provision is made such that a heat medium such as molten salt of a nitrate group may be circulated outside of the catalytic reaction tubes in the reactor apparatus. Such a type of reactor apparatus in the prior art is shown in FIG. 14.
In FIG. 14, a plurality of reaction tubes (heat transfer tubes) 1 packed with a catalyst and disposed in parallel to one another are fixed by upper and lower header plates 2. A heat medium serving as shell side fluid is introduced into a reactor shell 11 through an inlet nozzle 3 at the lower portion of the reactor shell 11, and after reaction heat has been recovered, the heat medium is discharged through an outlet nozzle 4 at the upper portion of the reactor shell 11. At that time, in order to improve the heat transfer performance of the heat medium, a plurality of baffle plates 5 are disposed within the reactor shell 11. The arrangement is such that raw material gas formed by mixing heated fluid propylene with air may flow into the reaction tubes 1 from the above through a nozzle 6, and after acrylic acid has been produced in the tubes 1 it is discharged through a nozzle 7.
In the above-described reactor apparatus in the prior art, for the purpose of enhancing a heat-exchange proportion of the shell side fluid, baffle plates or rods as shown in FIGS. 15, 16 and 17 were disposed.
FIG. 15 shows a most generally used baffle plate of a partly broken circular shape. Partly broken circular plates 5a and 5a' as shown in FIGS. 15(A) and 15(B) are alternately disposed in the direction of flow of the shell side fluid. In the case where such types of baffle plates are applied to a large-sized heat transfer apparatus in which the a number of heat transfer tubes is large and the tube length are large, the following problems are involved:
(1) The heat medium forming the shell side fluid would flow transversely of the heat transfer tubes in the respective flow passageways formed by the respective partly broken circular plates and, having alternately diverted flow directions, and so the a flow resistance of the shell side fluid is extremely increased. This means that a high amount of energy is consumed for circulation of the heat medium.
(2) Flow velocity distribution of the shell side fluid becomes uneven. In other words, there is a location where the flow velocity in the axial direction of the reactor shell is large, a location where a flow velocity in the radial direction is large, and a location where the fluid stagnates. Consequently, a large distribution is produced in the shell side heat transfer coefficient. Accordingly uneven distribution would arise in the catalyst reaction temperature, and hence degradation of the catalyst would be quickened and unevenness would arise also in the reaction speed, and the efficiency would be lowered.
FIG. 16 shows another example of the baffle plates, and in this example an annular (doughnut-shaped) plates 5b and circular plates 5b' are alternately disposed along the flow direction of the shell side fluid, that is, circular-/annular-shaped baffle plates are used. In the case of this type of baffle plates, although the pressure loss is reduced as compared to the above-described baffle plates of partly broken circular shape in FIG. 15, the above-mentioned problem (2) is not yet resolved. A distribution is produced in the shell side heat transfer coefficient, and so, this construction is unfavorable, for instance, for a reactor packed with a catalyst having a high temperature-dependent characteristic.
On the other hand, as a type for making the reduction of pressure loss and equalization of the heat transfer coefficient possible, rod baffles as shown in FIG. 17 are known. In this structure, reactor tubes (heat transfer tubes) 1 are supported by four kinds of rods 01-04 mounted to baffle rings 01A-01D, and little disturbance is produced in the shell side fluid flowing in a parallel flow. However, when this is applied to the above-described large-sized reactor or the like, new the following problems arise:
(1) From a structural requirement, the reactor tubes must be disposed in a square array within a reactor apparatus shell 06 as shown in FIG. 17(A), hence the shell diameter becomes large as compared to a triangular array, so that the flow velocity of the fluid is lowered, and accordingly the degree of improvement in the heat transfer coefficient is small.
(2) In order to maintain a heat transfer coefficient, it is necessary to choose small rod arrangement interval, and so, in a reactor having a large reactor tube length the number of rods is extremely increased.
(3) In a large-sized reactor having a large shell diameter, due to bending of the rods, manufacture and assembly become difficult.
(4) Since the heat medium forming a shell side fluid flows in a perfectly parallel flow, in the event that the amount of generated reaction heat in a certain reactor tube should be increased, the temperature rise of the heat medium covering that reactor tube would increase and would act in the direction of further increasing the temperature in the reactor tube, that is, thermal self-stability is not present.